Need help on how to approach my situation

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My friend has an inivida exhaust system on his subaru sti. The up pipe section keeps cracking. The first time it cracked he brought it to me to fix it and now it cracked again.  anyone know why it keeps cracking? is it my welding or is it the material, such as the metal being a low grade type? Its been awhile since i welded and just need some input on how to go about this. I am thinking about grinding the welds down completely and rewelding it, but do i have to grind the whole weld down or just the section where it cracked? There is no flex section in this exhaust system, so i am thinking that having one will help reduce alot of the stress for this uppipe. this is Stainless steel metal.thanks,BrentLast edited by disepyon; 12-21-2009 at 02:29 PM.
Reply:here are some update pictures i took after wire wheeling it.now this isnt a crack, its an indention from something. thought it was at first when it was dirty. hard to take good picture of it. Though it could still lead to cracking maybe i guess.
Reply:update pictures again.whats marked are the only problems with this sideThis is the other side with the markings on it.here is the inside of this side of the tube crack.again, the outside.
Reply:I would just grind out the cracked sections and get some good solid weld in there, but you should definetly have some type of flex/vibration absorber in there or it will continue to crack. Good luck, Tater
Reply:Originally Posted by PatatoheadI would just grind out the cracked sections and get some good solid weld in there, but you should definetly have some type of flex/vibration absorber in there or it will continue to crack. Good luck, Tater
Reply:x2 on getting some vibration isolation installed.  I also noticed that there are some grinding marks in the tube, right next to the upper toe line of the weld.  Those grinding marks are good places for fatigue cracks to start.If I had to guess, I'd say that the end of the tube was thinned down in the past, and that the vibrations are causing cracks to start and grow from those grinding marks.  The upper toe line is likely the thinnest material and the natural place for a crack to grow.Welding overtop all those grinding marks isn't practical for a bunch of reasons.  If I were making this repair, I'd cut away some of the original tube, and splice in a new piece.Since this is stainless you need to think about back purging the welds to prevent sugaring.  It would help if you could figure out what grade of stainless this stuff is.  Cheap(er) exhaust components can be 409 stainless, which is magnetic.  If a magnet doesn't stick, you've likely got a better grade of stainless, like 304/308/316.  I'd try and weld these with 316L filler metal, and good back purging procedure.Benson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:one more thought, I'd take the time, using the least abrasive method you can find, and clean all the soot and exhaust residue back at least 1" from the weld area, inside and outside.  That soot, if it gets in the weld puddle can contribute carbon to the weldment.  Raising the carbon level in the weld will embrittle the weld and degrade the corrosion resistance of the weld.Benson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:I'm assuming this was tig...What is the material and what filler did you use?The tube looks pretty thin while the flange is fairly thick. Maybe it needed pre-heat on the flange and use lower amps for the weld and some post heat to keep the tube from cooling too fast?MM350P/Python/Q300MM175/Q300DialarcHFHTP MIG200PowCon300SMHypertherm380ThermalArc185Purox oaF350CrewCab4x4LoadNGo utilitybedBobcat250XMT304/Optima/SpoolmaticSuitcase12RC/Q300Suitcase8RC/Q400Passport/Q300Smith op
Reply:thanks alot A_DAB_will_do, very well informative. I tried sticking a magnet on the tubing and it wouldnt stick, however the flange was very magnetic, i think its mild steel.When i welded this before i did back purge. ill be sure to do that again of course.ill ask my friend to think about buying a flex section. If he doesnt then i guess there isnt much to tell him other than it will keep on cracking and to repair it like you said, cutting out a section on each side of the tube and welding in new tubing and to have the surface of the flanges re done.
Reply:Originally Posted by DesertRider33I'm assuming this was tig...What is the material and what filler did you use?The tube looks pretty thin while the flange is fairly thick. Maybe it needed pre-heat on the flange and use lower amps for the weld and some post heat to keep the tube from cooling too fast?
Reply:IMHO the heating and cooling is the cause of the "flex" and that until you find a way to eliminate the heating and cooling cycles of the engine this will continue to crack. Better prep might slow down the time between the cracks reappearing but they will not go away indefinitely. At some point the tube will reach a point that it will never sustain it self. It will have work and heat hardened to the point it will not be weldable.
Reply:Could there have been some undercut at the leg of the weld that cracked during normal vibrations?
Reply:Originally Posted by Fat BastardIMHO the heating and cooling is the cause of the "flex" and that until you find a way to eliminate the heating and cooling cycles of the engine this will continue to crack. Better prep might slow down the time between the cracks reappearing but they will not go away indefinitely. At some point the tube will reach a point that it will never sustain it self. It will have work and heat hardened to the point it will not be weldable.
Reply:I would put money on the fact that the tubing is 300 series stainless, but the flange is crap steel. I would cut it off and fab a stainless flange. I know it's not an easy fix, but it beats repeat business due to failure (that don't pay).Last edited by papabear; 12-21-2009 at 11:34 PM."SOUTHPAW" A wise person learns from another persons mistakes;A smart person learns from their own mistakes;But, a stupid person.............never learns.
Reply:I would go with the dissimilar metals AND the flex. I have seen 4  guys try to fab up stainless headers and such and face the same issues. It came down to that 3 out the 4 times. Can't help you on the 4th! No idea - he threw em away! Could have been something else though.Just another clown trying to be cool
Reply:Two disimilar metals expanding and contracting at different rates every time the exhaust warms up or cools down, deffinately a problem. Not knowing that the first time you fixed it, and thus not using the right filler, also a deffinate problem. I second the notion of fabing a new plate, and I think you have enough votes for installing a flex pipe.
Reply:Originally Posted by disepyonthanks alot A_DAB_will_do, very well informative. I tried sticking a magnet on the tubing and it wouldnt stick, however the flange was very magnetic, i think its mild steel.When i welded this before i did back purge. ill be sure to do that again of course.ill ask my friend to think about buying a flex section. If he doesnt then i guess there isnt much to tell him other than it will keep on cracking and to repair it like you said, cutting out a section on each side of the tube and welding in new tubing and to have the surface of the flanges re done.
Reply:Hey guys,Thanks for all your input and different opinions about this problem.I decided to take this to a local fabricator i know, good guy, call him T Fab.  Rather give it to him since he knows what to do with it and has alot more tools than me and proper tools too.  He pretty much said alot of things you all said and what i thought to do with it. My friend still does not want to put the flex section in it, but he said he will do it some time later.I will be sure to post up pics when its done.Edit: Im pretty sure i would of known better to use the correct rod for welding on stainless and mild steel when i welded it, but damn, i could be wrong and used another type, just been so long, cant remember.Last edited by disepyon; 12-23-2009 at 12:00 AM.
Reply:Disepyon, I also have a STi and have never had a problem with seeing up-pipes crack like that. I had a auto-speed up-pipe that came with a flex joint in it (like the factory up-pipe) but recently got rid of it in favor of a 3 bolt up-pipe that is solid welded.But as you mentioned if the flange is MS, and the tubing is 304 you need to use 309L filler. Although that does seem weird that invidia would make a half and half up-pipe, unless it was a mistake. Have you thought about making some type of bracing for the top up-pipe flange or maybe making some small gussetts?